From: Paul Furman on
Wilba wrote:
> Paul Furman wrote:
>> Wilba wrote:
>>> Paul Furman wrote:
>>>> Wilba wrote:
>>>>> What do you see in this arrangement, back-focus or front-focus?
>>>>>
>>>>> Camera Subject Plane of focus
>>>> Back focus.
>>>>
>>>>> And therefore this is...?
>>>>>
>>>>> Camera Plane of focus Subject
>>>> Front focus.
>>> Excellent. So here we go.
>>>
>>> C = Camera,
>>> POF = Plane of Focus,
>>> DOF = Depth of Field,
>>> S = Subject.
>>>
>>> When the plane of focus is a long way (i.e. much greater than the DOF,
>>> >>DOF) in front of the subject -
>>>
>>> C POF<------ >>DOF ------>S
>>>
>>> I call that "near focus". You can think of it as initial gross
>>> front-focus.
>>>
>>> If I start like that, autofocus and beep focus both put the plane of
>>> focus coincident with the subject. Lovely.
>>>
>>> When the plane of focus is a long way behind the subject -
>>>
>>> C S<------ >>DOF ------>POF
>>>
>>> I call that "far focus". You can think of it as initial gross back-focus.
>>>
>>> If I start like that, autofocus and beep focus both put the plane of
>>> focus just outside the DOF (>DOF/2) on the _front_ side of the subject -
>>>
>>> C POF<-- >DOF/2 -->S
>>>
>>> Summary - initial gross front-focus results in optimal focus, and initial
>>> gross back-focus results in _front-focus_.
>>>
>>> This outcome -
>>>
>>> C S POF
>>>
>>> never occurs with my gear in my tests using autofocus or beep-focus.
>>>
>>> Let me know if that doesn't make sense. :- )
>> That makes sense but doesn't match your previous diagram:
>
> Yeah, but that's _your_ diagram, which is wrong. :- )

Well hell.
Maybe it is focus shift then.

> Here's mine again and we'll go through it line by line. Constant width font
> required for scale. Disregard any previous misunderstandings - these are the
> results. :- )
>
> --------------------------------------
>
> Beep Test (focus by changing the sensor to subject distance until the system
> beeps to confirm focus)
>
> Sensor to plane of focus distance -
> |< 430mm >|
> (The lens remains focussed at 430mm through the beep test.)
>
> Theoretical DOF -
> | |< 2.2mm >|< 2.2mm >|
> (The calculated DOF goes from 427.8 to 432.2mm.)
>
> Sensor to subject distance you get with an initial near-side focus -
> |< 430mm >|
> (Start with the subject 440mm from the sensor and decrease that distance.
> When the system beeps the subject is perfectly in focus.)
>
> Sensor to subject distance you get with an initial far-side focus -
> |< 433mm >|
> (Start with the subject 420mm from the sensor and increase that distance.
> When the system beeps the subject is 3mm further away than the plane of
> focus, i.e. front-focus.)
>
> The "beep band" -
> | |< 3.0mm >|
> (The system will confirm focus over this range of sensor to subject
> distances, for a plane of focus 430mm from the sensor.)
>
> Outside the DOF -
> | |<0.8mm>|
> (In the initial far-side focus case, the subject ends up 0.8mm further from
> the sensor than the far limit of the theoretical DOF.)
>
> --------------------------------------
>
> AF Test
>
> Sensor to subject distance -
> |< 430mm >|
> (The sensor to subject distance remains the same throughout the AF test.)
>
> Sensor to plane of focus distance you get with an initial near-side focus -
> |< 430mm >|
> (Start with the lens focussed at 420mm from the sensor and invoke AF. When
> the system beeps the subject is perfectly in focus.)
>
> Sensor to plane of focus distance you get with an initial far-side focus -
> |< 427mm >|
> (Start with the lens focussed at 440mm from the sensor and invoke AF. When
> the system beeps the plane of focus is 3mm short of the subject, i.e.
> front-focus.)
>
> Theoretical DOF for an initial far-side focus -
> | |< 2.2mm >|< 2.2mm >|
> (The calculated DOF goes from 424.8 to 429.2mm.)
>
> The "beep band" -
> | |< 3.0mm >|
> (The system will confirm focus over this range of sensor to plane of focus
> distances, for a subject 430mm from the sensor.)
>
> Outside the DOF -
> | |<0.8mm>|
> (In the initial far-side focus case, the subject ends up 0.8mm further from
> the sensor than the far limit of the theoretical DOF.)
>
> --------------------------------------
>
>


--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam
From: Wilba on
Chris Malcolm wrote:
> Wilba wrote:
>> Chris Malcolm wrote:
>>> Wilba wrote:
>>>> Paul Furman wrote:
>>>>> Wilba wrote:
>>>>>> Paul Furman wrote:
>>>>>>>
>>>>>>> The wider rays from f/1.8 won't be seen by the AF sensor. It's view
>>>>>>> comes through a system that only gathers narrower f/2.8 angled rays.
>>>>>>>
>>>>>>> The wider rays will get clipped, bouncing around side surfaces,
>>>>>>> never
>>>>>>> reaching the AF sensor.
>>>>>>
>>>>>> I guess you're saying that the effective size of an AF sensor's
>>>>>> virtual
>>>>>> aperture doesn't change with the lens aperture, which is fair enough,
>>>>>> because that's part of the explanation of why there is no ARFD
>>>>>> occurring in the AF system. Still, my system's performance meets that
>>>>>> standard (from an initial near side focus), so I'm not concerned
>>>>>> about
>>>>>> that aspect.
>>>>>>
>>>>>> It would be interesting to get a better idea of the effective size of
>>>>>> an AF sensor's virtual aperture (I think I read somewhere that it
>>>>>> might
>>>>>> be something like f/11, but I can't recall where). As long as we're
>>>>>> clear
>>>>>> that "f/2.8" doesn't refer to that. :- )
>>>>>
>>>>> Heh, now I don't know what you're talking about :-)
>>>>
>>>> Isn't this fun!? :- )
>>>>
>>>> See figure 13 of http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf
>>>> (but read your way down to it from the beginning so that it makes
>>>> sense).
>>>>
>>>> The light that reaches the AF sensor effectively comes through a
>>>> virtual
>>>> aperture in the lens ("virtual AF apertures" in figure 13), as we have
>>>> been discussing for quite some time. If we knew the diameter of one of
>>>> those virtual apertures, we could work out the f-number for it. IIRC,
>>>> I read somewhere that it is typically something like f/11. As long as
>>>> we
>>>> are clear that the "f/2.8" in "f/2.8 high-precision AF sensor" does not
>>>> refer
>>>> to the size of the virtual AF aperture.
>>>
>>> "As we have all been discussing for some time"?
>>>
>>> Since that document is the only one I've ever seen which uses "virtual
>>> AF aperture" in that sense, and all the other uses I've seen, by
>>> camera makers, reviewers, etc., have used it in the other sense of the
>>> width of lens the pair stretch across, I'm pretty sure everyone (except
>>> you) in this discussion has been using it in that sense too.
>>
>> Why won't you provided references to any such documents?
>
> "Back and front focus errors are common with very fast lenses (and
> others). The AF sensors in all DSLRs operate at effective apertures
> between f/2.8 and f/7.1, and 'see' the focus point as if the lens was
> stopped down to the exact virtual f-stop the sensor in use
> imposes. When spherical aberration causes a shift in focus on
> stop-down, both wider and smaller apertures will no longer focus on
> the targeted plane. This is corrected in camera calibration, general
> or lens-specific."
>
> This quote comes from the section "Focus Accuracy" in this article in
> the British Journal of Photography.
>
> http://www.bjp-online.com/public/showPage.html?page=837772

Excellent! So, now that you have some corroboration for that idea, please
remind me what you're claiming as a consequence of it.


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>>
>> let's say for argument's sake the "AF field stops" were controlling the
>> rays that reach the AF sensor, what would be the relevance,
>> to what, and in what way?
>
> No particular relevance to the focus shift issue :-)

No worries. Thanks for joining in a stimulating and challenging discussion.
:- )


From: Wilba on
Paul Furman wrote:
> Chris Malcolm wrote:
>>
>> "Back and front focus errors are common with very fast lenses (and
>> others). The AF sensors in all DSLRs operate at effective apertures
>> between f/2.8 and f/7.1, and 'see' the focus point as if the lens was
>> stopped down to the exact virtual f-stop the sensor in use
>> imposes. When spherical aberration causes a shift in focus on
>> stop-down, both wider and smaller apertures will no longer focus on
>> the targeted plane. This is corrected in camera calibration, general
>> or lens-specific."
>>
>> This quote comes from the section "Focus Accuracy" in this article in
>> the British Journal of Photography.
>>
>> http://www.bjp-online.com/public/showPage.html?page=837772
>
> That makes sense. The AF system only sees things at the designed aperture.

I can see we're not going to agree on this, but I'd say it like, the AF
system only sees things through an effective aperture, whose size is a
consequence of the design of elements such as the prisms.


From: Wilba on
Paul Furman wrote:
> Wilba wrote:
>> Paul Furman wrote:
>>> Wilba wrote:
>>>> Paul Furman wrote:
>>>>> Wilba wrote:
>>>>>>
>>>>>> What do you see in this arrangement, back-focus or front-focus?
>>>>>>
>>>>>> Camera Subject Plane of focus
>>>>>
>>>>> Back focus.
>>>>>
>>>>>> And therefore this is...?
>>>>>>
>>>>>> Camera Plane of focus Subject
>>>>>
>>>>> Front focus.
>>>>
>>>> Excellent. So here we go.
>>>>
>>>> C = Camera,
>>>> POF = Plane of Focus,
>>>> DOF = Depth of Field,
>>>> S = Subject.
>>>>
>>>> When the plane of focus is a long way (i.e. much greater than the DOF,
>>>> >>DOF) in front of the subject -
>>>>
>>>> C POF<------ >>DOF ------>S
>>>>
>>>> I call that "near focus". You can think of it as initial gross
>>>> front-focus.
>>>>
>>>> If I start like that, autofocus and beep focus both put the plane of
>>>> focus coincident with the subject. Lovely.
>>>>
>>>> When the plane of focus is a long way behind the subject -
>>>>
>>>> C S<------ >>DOF ------>POF
>>>>
>>>> I call that "far focus". You can think of it as initial gross
>>>> back-focus.
>>>>
>>>> If I start like that, autofocus and beep focus both put the plane of
>>>> focus just outside the DOF (>DOF/2) on the _front_ side of the
>>>> subject -
>>>>
>>>> C POF<-- >DOF/2 -->S
>>>>
>>>> Summary - initial gross front-focus results in optimal focus, and
>>>> initial gross back-focus results in _front-focus_.
>>>>
>>>> This outcome -
>>>>
>>>> C S POF
>>>>
>>>> never occurs with my gear in my tests using autofocus or beep-focus.
>>>>
>>>> Let me know if that doesn't make sense. :- )
>>>
>>> That makes sense but doesn't match your previous diagram:
>>
>> Yeah, but that's _your_ diagram, which is wrong. :- )
>
> Well hell.
> Maybe it is focus shift then.

No! No! No! No! :- )

It's all about the beep band (see below). From one direction focus is
confirmed in the right place, and from the other it's confirmed in the wrong
place. Where is the focus shift in that?


>> Here's mine again and we'll go through it line by line. Constant width
>> font required for scale. Disregard any previous misunderstandings - these
>> are the results. :- )
>>
>> --------------------------------------
>>
>> Beep Test (focus by changing the sensor to subject distance until the
>> system beeps to confirm focus)
>>
>> Sensor to plane of focus distance -
>> |< 430mm >|
>> (The lens remains focussed at 430mm through the beep test.)
>>
>> Theoretical DOF -
>> | |< 2.2mm >|< 2.2mm >|
>> (The calculated DOF goes from 427.8 to 432.2mm.)
>>
>> Sensor to subject distance you get with an initial near-side focus -
>> |< 430mm >|
>> (Start with the subject 440mm from the sensor and decrease that distance.
>> When the system beeps the subject is perfectly in focus.)
>>
>> Sensor to subject distance you get with an initial far-side focus -
>> |< 433mm >|
>> (Start with the subject 420mm from the sensor and increase that distance.
>> When the system beeps the subject is 3mm further away than the plane of
>> focus, i.e. front-focus.)
>>
>> The "beep band" -
>> | |< 3.0mm >|
>> (The system will confirm focus over this range of sensor to subject
>> distances, for a plane of focus 430mm from the sensor.)
>>
>> Outside the DOF -
>> | |<0.8mm>|
>> (In the initial far-side focus case, the subject ends up 0.8mm further
>> from the sensor than the far limit of the theoretical DOF.)
>>
>> --------------------------------------
>>
>> AF Test
>>
>> Sensor to subject distance -
>> |< 430mm >|
>> (The sensor to subject distance remains the same throughout the AF test.)
>>
>> Sensor to plane of focus distance you get with an initial near-side
>> focus -
>> |< 430mm >|
>> (Start with the lens focussed at 420mm from the sensor and invoke AF.
>> When the system beeps the subject is perfectly in focus.)
>>
>> Sensor to plane of focus distance you get with an initial far-side
>> focus -
>> |< 427mm >|
>> (Start with the lens focussed at 440mm from the sensor and invoke AF.
>> When the system beeps the plane of focus is 3mm short of the subject,
>> i.e. front-focus.)
>>
>> Theoretical DOF for an initial far-side focus -
>> | |< 2.2mm >|< 2.2mm >|
>> (The calculated DOF goes from 424.8 to 429.2mm.)
>>
>> The "beep band" -
>> | |< 3.0mm >|
>> (The system will confirm focus over this range of sensor to plane of
>> focus distances, for a subject 430mm from the sensor.)
>>
>> Outside the DOF -
>> | |<0.8mm>|
>> (In the initial far-side focus case, the subject ends up 0.8mm further
>> from the sensor than the far limit of the theoretical DOF.)